1. Field of the Invention
This invention relates generally to amplifiers and, more particularly, to a class D audio amplifier.
2. Background Art
Power-hungry linear amplifiers have long dominated the world of audio. But, as portable-electronics consumers demand smaller devices with longer battery life, designers are looking to replace conventional linear amplifiers with high-efficiency, fully-integrated devices. Although class AB amplifiers continue to dominate the world of audio, switch-mode amplifiers are gradually conquering the consumer market, as they provide a good balance between efficiency and distortion.
Integrated circuit technologies favor the implementation of switch-mode audio amplifiers. They also provide means of integrating complex circuits for digital signal processing and power management in a chip.
The power efficiency of the driver is dominated by the power stage, and analog amplifiers (e.g., class A, class B and class AB amplifiers) tend to be inefficient. In a switch-mode amplifier (i.e., class D amplifier), the power stage is feed with a modulated signal from a pulse width modulator (PWM). This greatly increases the efficiency of the power stage.
Audio power-amplifiers based on PWM techniques and class D output stages are well-known in the art as an alternative to conventional AB amplifiers. Such designs use analog signal processing to form the PWM signal by comparing the audio signal with a high frequency saw tooth or triangle signal that acts as a carrier signal.
The transistors in a traditional class AB amplifier operate in the linear region, acting as variable resistor network between the supply and the load with the voltage that is dropped across the transistors being lost as heat. In contrast, the output transistors of a class D amplifier switch from full OFF to full ON (saturated) and then back again, spending very little time in the intermediate linear region. This results in much lower power loss in the switching transistors of the class D amplifier. Ideally, the switches of the class D amplifier dissipate no heat energy with a theoretical efficiency of 100% as opposed to the class B amplifier which has a theoretical maximum efficiency of 78%.
The advantages of class D drivers include, therefore, a high efficiency and small dimensions due to reduced needs for heat sinking. Also, the transfer characteristics of class D drivers output stage is far less prone to change with temperature and process variation then a conventional open loop class AB stage that requires careful bias control and matching of complementary devices.
Many additional benefits arise when the class D amplifier it is used in an entirely digital signal processing chain to transform a digital pulse code modulated (PCM) signal into a PWM signal that directly drives the class D output stage. The resulting amplifier is a true digital power amplifier that acts as a digital to analog converter (DAC), can provide high output power, and does not rely on analog signal processing and amplification. The internal functions of such a switch-mode power DAC circuit include: converting the PCM input signal into a PWM signal; amplifying the pulse signal with a half/full Bridge of power transistors; and filtering the high frequency contents of the power-switched output signal to recover the baseband input signal by removing the carrier signal and respective switching components.
One simple low-pass filter demodulation process requires that the amplified pulse signal must have a baseband free of any distortion terms (harmonic and non-harmonic). A lot of effort have been taken to overcome the deficiencies of direct PCM to PWM conversion in order to minimize distortion terms in the baseband. The simplest conceptual architecture is impractical, requiring clock speed in order of GHz for an audio frequency amplifier. The incorporation of oversampling and noise-shaping techniques can reduce the clock speed down to several tens of MHz, but distortions due to the underlying modulation process are still problematic.
Another approach to reducing distortion is to modify or pre-process the input signal before it is applied to the modulator. The nature of pre-processing will define the spectral characteristics of the output PWM signal and its fidelity in band noise and distortion. There are reported results of direct PWM modulation with pre-processing that can simultaneous achieve high fidelity and high efficiency. Beyond the nature of pre-processing, there are also variations on the PWM modulation process. The main classes of direct PWM include natural PWM (NPWM), uniform sampling PWM (UPWM), linear approximated PWM (LPWM), polynomial approximated PWM (PNPWM), and weighted PWM (WPWM).
A further approach to pulse modulation, applies a sigma-delta modulator structure within a digital power amplifier strategy. Reported implementations use normal, synchronous modulators that require relatively high switching frequencies to obtain good fidelity but at the expense of power efficiency.
Moreover, despite good modulator designs, an open-loop output power stage will introduce significant distortion and noise. Any amplifier solution must take into account the fact that the power stage and demodulation filter are inherently analog. Their errors are difficult to predict and difficult to eliminate in the digital domain.
What is needed is a high quality PWM input signal that can reach 0 dBFS (decibels full scale) with a controlled and preferably fixed, carried frequency. The digital PWM modulators normally used for audio applications have distortion problems that can only be solved with complex pre-processing algorithms. The sigma-delta modulators have noise shaping capabilities but do not offer a fixed frequency output signal as is preferred for class D amplifiers. Also, only modulators with multi-bit outputs can reach 0 dBFS without stability and distortion issues, but a multi-bit signal is not suitable to feed a class D audio amplifier.
What is needed is a class D amplifier that overcomes these limitation of known class D amplifiers.